In carrying out the pyrogenous production of very finely dispersed oxides of a metal and/or a metalloid, volatile or evaporable compounds of the metal and/or the metalloid are fed together with a combustible or steam-forming gas and oxygen or oxygen containing gases either separately or in a mixture to a burner. At the same time the combustible and the oxygen containing gas are fed-in, in such a quantitative ratio, which guarantees both a complete combustion of the combustible gas as well as the hydrolysis of the evaporable compound of the metal and/or the metalloid.
Whenever inorganic or organic halogen compounds of the metal and/or of the metalloid are used, then the metalloxide and/or metalloid oxide are obtained together with a waste gas, containing halogen halide, which is separated in proper separating apparatus from the metal and/or metalloid oxide. Elementary halogen is formed in a secondary reaction. Depending on the conduction of the reaction conditions for the formation of the metal oxide and/or metalloid oxide, 6 to 10% by weight of elementary halogen develops, based on the quantity of halogen halide formed.
It has heretofore been known, in a pyrogenous process for the production of oxides of a metal and/or metalloid, to remove the developed elementary halogen, for example, chlorine, from the waste gas of the reaction, by reducing the formed elementary halogen with hydrogen during the cooling off of the reaction product below the reaction temperature of the hydrogen, with the oxygen contained in the waste gas of the reaction (German OS No. 25 33 925).
In this prior known process, wherein SiCl.sub.4 (silicon tetrachloride) is used as the starting material for the production of silicon dioxide, the mixture developed from reaction waste gas and silicon dioxide, passes within a relatively long cooling section through a temperature drop of 1000.degree. to 200.degree. C. during cooling off. The additional hydrogen is introduced into an area of the cooling section in which the reaction waste gases still have a temperature of 500.degree. to 700.degree. C. An addition of the hydrogen at a temperature above 700.degree. C. is not to be recommended, because a reaction of the hydrogen with the oxygen will occur at higher temperatures. The addition of the hydrogen at a temperature of below 500.degree. C. is likewise not to be recommended since there the reaction rate of the reaction speed of the hydrogen with the elementary chlorine is too slow.
The best results may be obtained, whenever the elementary hydrogen is introduced at a temperature between 550.degree. and 630.degree. C.
The precise point of introduction for the elementary hydrogen in the cooling section depends on the load, i.e., in case of changes--depending on production--of the flow velocity, the point of introduction for the elementary hydrogen is the cooling section must be shifted.
A pipe is used for the introduction of the hydrogen, which, in its length, corresponds to the diameter of the cooling section. This pipe, in its jacket, has two rows of bores through which the hydrogen is introduced. A cross-sectional drawing of such an introducing pipe is shown in FIG. 1.
However, the known process has the disadvantage that even in case of a small load changes the point of introduction for the hydrogen must be changed. It is a further disadvantage, that under certain circumstances the hydrogen reacts very violently with the elementary chlorine with formation of flames. Thus the reaction becomes uncontrollable and under certain circumstances it may lead to an impairment of the silicon dioxide contained in the reaction waste gas. A premature wear of the introducing pipes for the hydrogen is connected with that.